1. Field of the Invention
This invention relates generally to heating, ventilation and air conditioning systems, and more particularly relates to a method and apparatus for improving the efficiency of a heat pump heating and cooling system.
2. Description of the Related Art
According to the second law of thermodynamics, heat will not flow from a cool body to a warm body without the input of work. It is also known to heat and cool a house, office, apartment or other dwelling by “pumping” heat from a cool body to a warm body. This is accomplished, most typically, by cyclically compressing and expanding a fluid, and strategically removing thermal energy from, and adding thermal energy to, the fluid, where desired. A heat pump thus extracts thermal energy from one location, such as a warm home, and releases the thermal energy to the warmer outdoor air, for example to cool the home in the summer. Additionally, the heat pump system can be used to extract thermal energy from the outdoor air during cooler months, and release the thermal energy indoors to heat the dwelling.
Because they move thermal energy from a cold source (e.g., the outdoor air) to a hot heatsink (e.g., the indoor furnace coil), heat pumps require energy to perform the work necessary, and this energy is commonly electrical energy used to drive an electric motor. The amount of thermal energy deposited at the warm side is less than the amount of energy taken from the cool side by an amount equal to the work performed. Heat pumps consume less energy to heat a home in many climates than other conventional heating systems.
One common type of heat pump works by exploiting the physical properties of a fluid refrigerant. The working fluid is circulated through the system, which includes two heat exchangers, a compressor and an expansion valve. The working fluid, in its gaseous state, is pressurized and circulated through the system by the compressor. On the discharge side of the compressor, the hot and highly pressurized gas is cooled in one of the heat exchangers (called a “condenser”) until it condenses into a high pressure, moderate temperature liquid. The condensed refrigerant then passes through a pressure-lowering device such as an expansion valve, a capillary tube, or a work-extracting device such as a turbine. The low pressure, barely liquid (saturated liquid) refrigerant then passes through another of the heat exchangers (called an “evaporator”) where the refrigerant evaporates into a gas via heat absorption. The refrigerant then returns to the compressor and the cycle is repeated.
In such a system it is essential that the fluid reach a sufficiently high temperature when compressed, since the second law of thermodynamics prevents heat from flowing from a cold fluid to a hot heatsink. Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or heat cannot flow from the cold region into the fluid. In particular, the pressure difference must be great enough for the fluid to condense at the hot side and still evaporate in the lower pressure region at the cold side. The greater the temperature difference, the greater the required pressure difference, and consequentially more energy is needed to compress the fluid. Thus as with all heat pumps, the energy efficiency (amount of heat moved per unit of work required to drive the pump) decreases with increasing temperature difference between the hot side and the cold side.
This temperature difference is rarely an issue in the hottest months, when the desired indoor temperature rarely is more than 40 degrees different from the outdoor temperature. However, in winter in cold climates, the outdoor temperature can be 70 degrees Fahrenheit lower than the desired indoor temperature, and can reach 100 degrees difference. Under these circumstances, heat pumps are not efficient enough to justify their use.
This temperature difference can be reduced if one uses a geothermal system, in which the relatively constant temperature of the earth is used to pre-heat and/or pre-cool water, thereby reducing the temperature difference between the air in the home and the fluid from which energy is taken, or to which energy is given. However, geothermal systems are very expensive to install, making them accessible only to those with the means to invest substantial amount at the outset in exchange for decreased heating and cooling bills in the future.
Additionally, it is a frequent complaint of users of forced air heat pump systems that the temperature of the air coming out of the air ducts during cooler months is not sufficiently warm to create the comfort level normally desired in a dwelling. The temperature of the air at the air ducts is typically only a few degrees warmer than the ambient air. This is in contrast to the temperature at the air ducts of fossil fuel heating systems, which can be 20 to 40 degrees warmer than ambient air.
Therefore, there is a need for a system that has the efficiency of a heat pump system, but which avoids the problem of perceived cold air coming out of the air ducts.